Removal of body hair is conventionally achieved with apparatus termed an epilator, by injecting a pulse of RF energy into a hair follicle. A conventional epilator, hereinafter termed an epilator of the type described, comprises a timing circuit for gating the output of an RF oscillator into a probe which has a needle adapted to be inserted into a hair follicle. The RF energy radiated from the needle is dissipated as heat in the tissue surrounding the needle. The heat so produced causes local destruction of tissue at the hair follicle; and if the needle is properly sited relative to the hair follicle, and if sufficient energy is radiated by the probe, the resulting destruction of the follicle will be permanent.
In using a conventional epilator, the operator grips a hair to be removed with a tweezer, and inserts the needle on the probe into the hair follicle. From past experience in using an epilator, and with past experience in dealing with a particular subject, the operator selects a pulse duration (the amplitude is usually maintained constant) and then actuates a foot switch which turns on the epilator and delivers to the probe a pulse of predetermined amplitude and duration. If the proper time has been estimated by the operator, the injected RF pulse will permanently destroy the hair follicle, and the subject will feel no significant pain or discomfort. On the other hand, if less than the proper time has been set, the hair follicle will be damaged not permanently destroyed; and the process will have to be repeated. On the other hand, if more than the proper time has been selected, the hair follicle may be destroyed permanently but at the expense of a considerable amount of pain and discomfort. Furthermore, the proper time interval for a pulse which will result in permanent destruction of a hair follicle at one site without significant pain or discomfort to a subject is likely to be different for another site in the same subject. Thus, experience shows that the proper time interval varies from one location to another on the same subject, as well as from subject to subject. For this reason, a great deal of operator skill and also luck are involved in utilizing a conventional epilator for permanently destroying hair follicles without significant pain or discomfort to a subject.
The problem apparently arises because the impedance seen by the probe may vary widely from one skin location to another as well as from one subject to another. In general, the impedance of a load to an RF source of energy will depend on the conductivity of the load, the reactance (capacitive or inductive) of the load, and the coupling between the source and the load. Where the RF source is a needle imbedded in tissue, the impedance presented to the needle will depend on the amount of moisture in the tissue, blood salinity, depth of insertion, thickness of the epidermal layer, etc. Such impedance is known to vary greatly from one insertion of the needle of a probe to another in the same subject, from one skin area to another and from one subject to another.
On the average, the impedance of tissue to an RF pulse of about 13 MHz (which is the diathermy frequency reserved by the F.C.C.) is about 2000 ohms; but it has been found that a power input of from 2.5-7 watts for about 100 msec into this impedance will be adequate to permanently destroy a hair follicle without significant pain or discomfort to the subject. Thus, it is conventional to design the probe of an epilator so that its impedance is about 2000 ohms, and to provide for a variable output of from 2.5 to 7 watts. If such a probe were used under the conditions that an impedance mismatch exists (i.e., the impedance as seen by the probe when the needle is inserted into a hair follicle is significantly different from the design value), then the required energy to permanently destroy the follicle without pain or discomfort to the subject will not be delivered to the tissue, resulting either in non-permanent destruction of the hair follicle or a significant amount of pain to the subject. For example, if the impedance presented to the probe is greater than 2000 ohms, then less power will be dissipated in the tissue and the balance will be reflected back into the epilator unbalancing the drive and degrading the output power and waveshape. As a consequence, treatment with a conventional epilator is often ineffectual and painful.
It is therefore an object of the present invention of provide an new and improved method of and apparatus for removing hair using an epilator wherein the above described deficiencies of the prior art are significantly reduced or eliminated.